How Birds Cooperate to Defeat Cuckoos

Understanding why evolution drove such behaviour remains controversial. Some studies have linked its occurrence with variable and unpredictable environmental conditions, while others have linked it to stable and predictable conditions.
Cooperative breeding
[Pin It] Three cooperatively breeding superb fairy-wrens.
Credit: Alecia Carter.
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We thought it might have something to do with defending their nests against brood parasitism, a behaviour where other birds to raise your babies. Brood parasitism is most easily recognisable among cuckoo birds, who never build their own nest or raise their own offspring. Instead they lay their eggs in the nests of birds from other species, and leave the substantial task of raising their chick to the unsuspecting host.

We suspected that if larger cooperative breeding groups are better able to defend their nests against brood parasitism, these breeding systems may be evolutionarily linked.

To investigate this question we first looked at the global distribution of cooperative breeding and brood parasitic bird species. If these breeding systems are linked, brood parasites and cooperative breeders should live in the same areas.

We found that brood parasites and cooperative breeders are found in the same places around the world, with particularly high concentrations in sub-Saharan Africa and Australasia. So next we looked at whether hosts of brood parasites are more likely to be cooperative breeders. We conducted analyses focusing on Australia and South Africa as the species in these areas are particularly well understood. Again, we found that hosts of brood parasites were more likely to be cooperative breeders than non-host species in both these areas.

However, these analyses do not tell us whether cooperative breeders are being chosen by brood parasites, or if brood parasitism is driving species to become cooperative. To gain insight into this question, we conducted a detailed study of the superb fairy-wren (Malurus cyaneus). This bird cooperatively breeds, and is also a host of the Horsfield’s bronze-cuckoo (Chalcites basalis) in South-Eastern Australia.